[GHC] #14935: Vary default RTS settings so that performance does not degrade with increasing number of capabilities

[GHC]

#14935: Vary default RTS settings so that performance does not degrade with
increasing number of capabilities
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           Reporter:  YitzGale       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 When the number of capabilities increases to 32 or more, which is common
 nowadays, performance of GHC-compiled applications, hence also of GHC
 itself, begins to degrade considerably with default RTS settings.

 It should not be required to optimize RTS settings manually to get
 applications to be usable. By default, GHC should use at least sane RTS
 settings appropriate for the number of capabilities, even if not
 optimized.

 See:

 https://www.reddit.com/r/haskell/comments/83e6dq/need_advice_on_compile_and_runtime_options_for/

 for some of the RTS settings you need to change to get things working.

 Here is a sample use case - we needed to increase memory by a lot to work
 around #14928. We don't care about extra cores, but we get them
 automatically when we enlarge our EC2 instance to have enough memory. The
 compile is run by deployment engineers - non-Haskell-programmers - via
 build tools that normally do not give easy access to RTS options on the
 individual {{{ghc}}} commands that run during the build ({{{yesod keter}}}
 in this case). We just want the build to run no worse than if there were 4
 cores, or even 1 core.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14935>
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